1. Field of the Invention
This invention relates to an improved process for the preparation of high Mooney viscosity interpolymer elastomers of ethylene, vinyl acetate and reactive halogen-containing monomers, such as vinyl chloroacetate (VCA). The incorporated halogen-containing units serve as cure sites for vulcanization. More particularly, the process is directed to novel methods for introducing the vinyl chloroacetate cure site monomer during polymerization so that interpolymer elastomer products which exhibit increased tensile strength properties upon vulcanization are obtained.
2. Background of Invention
The production of ethylene-vinyl acetate-vinyl chloroacetate (E/VA/VCA) interpolymer elastomers is known in the art. For example, Kaizerman et al., in U.S. Pat. No. 3,972,857, prepare E/VA/VCA interpolymers by chemically reacting an ethylene-vinyl acetate (EVA) copolymer elastomer with chloroacetic acid in xylene solution using p-toluenesulfonic acid as the catalyst. Acetic acid, produced by acidolysis of the EVA copolymer by the chloroacetic acid, is removed by distillation and the EVA copolymer, now containing chemically-introduced VCA units, is obtained by precipitation with isopropyl alcohol. Elastomers containing from 0.2 to 4.5 weight percent chlorine which can be vulcanized with soap/sulfur cure systems are obtained.
Another method of producing soap/sulfur vulcanizable ethylene-vinyl acetate elastomer compositions is that of Chang et al. in U.S. Pat. No. 4,202,845. Chang et al. graft an acrylate ester monomer along with a vinyl chloroacetate cure site monomer onto an elastomeric EVA copolymer backbone.
As indicated, both of the above compositions which contain reactive halogen cure sites undergo soap/sulfur vulcanization to obtain useful elastomeric products. Typically, soap/sulfur vulcanization involves, in addition to elastomer and carbon black reinforcing agent, a soap such as sodium stearate or sodium 2-ethylhexanoate, sulfur, stearic acid and an antioxidant. No unsaturation is required in the elastomer for soap/sulfur vulcanization. While the exact mechanism is uncertain, there are indications that --S.sub.x -- crosslinks are introduced [Kaendler et al., Die Angewandte Makromolekulare Chemie, 29/30, 241(1973)]. In any event, soap/sulfur vulcanization differs fundamentally from conventional sulfur vulcanization where carbon-carbon double bond unsaturation must be present in the elastomer.
There are several disadvantages associated with the aforesaid chemical processes of Kaizerman et al. and Chang et al. for introducing cure sites into ethylene-vinyl acetate copolymer elastomers. First of all for Kaizerman et al., a multi-step procedure is involved which increases costs. The EVA first has to be synthesized, then dissolved in xylene, and thereafter reacted with chloroacetic acid in a relatively slow, complicated solvent process. The chemically modified product must then be recovered by precipitation with a nonsolvent, such as isopropyl alcohol, and dried. Recovery, separation and purification of solvent and non-solvent are required for commercial operation.
The Chang et al. graft polymerization is also a multi-step process and suffers from the attendant increases in cost. The EVA elastomer must first be synthesized and then grafted in a slurry process with a substantial amount of relatively expensive acrylate ester monomer, said graft process also incorporating the VCA cure sites into the finished graft composition.
It would be highly desirable and advantageous if a simple,low cost procedure for interpolymerizing ethylene and vinyl acetate with a comparatively small amount of VCA or other similar halogen-containing cure site monomer were available. It is, however, recognized among polymer chemists that VCA is a rather potent chain transfer agent. This propensity to chain transfer tends to limit the molecular weight of the interpolymers that can be produced using VCA as the cure site monomer. The aforesaid chain transfer character of the cure site monomer becomes even more of a problem at higher temperatures such as are employed for conventional high pressure polymerization processes. Thus, in order to produce sufficiently high molecular weight E/VA/VCA interpolymer elastomers, the polymerizations have typically been conducted at relatively low temperatures by redox-initiated polymerization in aqueous emulsion or solution.
One such method for the emulsion polymerization of ethylene, vinyl acetate and vinyl chloroacetate is disclosed by Becker et al. in U.S. Pat. No. 4,098,746. Becker et al. disclose that the polymerization may be carried out in a batch process or by feeding the monomers to the reaction medium. They also indicate the polymerization can be carried out at a constant, increasing, or decreasing ethylene pressure. There is, however, no disclosure by Becker et al. of what effect the manner or rate of varying ethylene pressure or monomer addition during the interpolymerization process might have on the properties of the interpolymer. Moreover, when Becker et al. add VCA monomer during the course of the polymerization, it is introduced as a dilute solution (up to about 10 weight percent) in vinyl acetate monomer, as such or as an aqueous emulsion. Addition of VCA monomer alone or at relatively high concentrations in vinyl acetate monomer is not suggested by Becker et al. Furthermore, Becker et al. do not employ soap/sulfur cure systems but rather utilize cross-linking agents, such as aminoplast resins, polyamines, polyamidoamines, or mixtures of formaldehyde and ammonia or amine, for cross-linking.
Heimberg in U.S. Pat. No. 4,287,329 discloses an emulsion polymerization process for synthesizing EVA elastomers of low gel content containing about 40% to about 70% by weight of vinyl acetate and having Mooney viscosities of about 30 to 80. In contrast to EVA elastomers of the art produced by high pressure polymerization processes conducted at relatively high temperatures and which seldom have Mooney viscosities much in excess of about 20 and as a result are soft and tacky and very difficult to handle and compound on rubber processing equipment, the high Mooney viscosity emulsion elastomers of Heimberg are less tacky and present much less difficulty in handling and compounding on the rubber mill.